This invention relates to an automatic locking device for linkages subjected to undesirable mechanical stresses, whichis particularly applicable for example to the opening release devices used in electrical switches.
It is known that a linkage subjected to mechanical stresses such as impact, vibration, acceleration etc. of sufficient intensity and suitable direction is able to undergo movement.
In many cases this movement is undesirable, and leads to the functioning of the system in which the linkage is inserted, the result being untimely and sometimes unacceptable operation.
Electrical switches installed in structures susceptible to high stress are usually provided with locking devices for the release linkage, in order to prevent involuntary opening of the contacts due to undesirable mechanical stress.
These locking devices are of mechanical type. One type comprises for example a ball elastically held between two jaws, to produce by the effect of impact a mechanical action which is able to lock the linkage for opening the switch contacts. In practice, on impact, the ball moves from its rest position and causes the two jaws to diverge, thus preventing movement of the release system and locking it.
However in the particular case of an overcurrent release device, when in this locked position even if a release control signal due to an overload acts simultaneously with the impact stress, it is possible for the release device to be unable to operate and cause opening of the switch contacts. This represents a drawback.
Furthermore, a negative characteristic typical of these known mechanical locking devices is their response time to mechanical stresses. This time is relatively high and is due to the need for the sensing mass (the ball) to undergo a finite displacement sufficient to move the mechanical locking elements (the two jaws).
This can be particularly disadvantageous in those applications which use electromagnetic releases with a switch opening device in the form of a solenoid which is retained in position by a permanent magnet field, and is operable by an electrical demagnetisation pulse.
Such opening solenoids are known to comprise an armature which operates the switch release lever. This armature is held in its "set" position by a permanent magnet and simultaneously loads an operating spring. A very small displacement of the armature from its "set" position is sufficient to enable the spring to prevail over the permanent magnet force, so as to cause the armature to move rapidly until it operates the switch release lever. In the absence of impact, such a displacement can be obtained merely by an electromagnetic pulse force acting against the permanent magnet force and generated by a coil energised by the output signal from overcurrent sensors.
It therefore follows that even impacts of a not particularly high intensity are able to cause the release device to involuntarily operate.
The intervention action of present-day mechanical locking devices is however not sufficiently rapid to anticipate the release device, because of the rapidity of the elastic snap-action of the armature in contrast to the high inertia of the mechanical locking device.
The ideal solution would be a locking device which is so rapid as to not enable the armature to make even a minimum movement, because if this were not the case once the effect of the impact and thus the intervention of the locking device has ceased, the armature, under the action of the spring, would continue its movement until it operated the release lever, even if a signal from the overcurrent sensors were not present.